Impossible USB mount in recovery mode

Hi,

when I boot in recovery mode and then open a root shell prompt, I can't mount my external USB HDD.

DMESG says only:
[ 2947.060085] usb 2-2: new high speed USB device number 5 using ehci_hcd

while in normal mode DMESG says:
[ 2947.060085] usb 2-2: new high speed USB device number 5 using ehci_hcd
[ 2947.265668] usbcore: registered new interface driver uas
[ 2947.282429] Initializing USB Mass Storage driver...
[ 2947.282555] scsi4 : usb-storage 2-2:1.0
[ 2947.282624] usbcore: registered new interface driver usb-storage
[ 2947.282626] USB Mass Storage support registered.
[ 2949.945283] scsi 4:0:0:0: Direct-Access Hitachi HTS542525K9SA00 PQ: 0 ANSI: 2 CCS
[ 2949.957766] sd 4:0:0:0: Attached scsi generic sg2 type 0
[ 2950.028870] sd 4:0:0:0: [sdb] 488397168 512-byte logical blocks: (250 GB/232 GiB)
[ 2950.029471] sd 4:0:0:0: [sdb] Write Protect is off
[ 2950.029480] sd 4:0:0:0: [sdb] Mode Sense: 00 38 00 00
[ 2950.029971] sd 4:0:0:0: [sdb] Asking for cache data failed
[ 2950.029981] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 2950.033979] sd 4:0:0:0: [sdb] Asking for cache data failed
[ 2950.033990] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 2950.059757] sdb: sdb1 sdb2
[ 2950.063694] sd 4:0:0:0: [sdb] Asking for cache data failed
[ 2950.063698] sd 4:0:0:0: [sdb] Assuming drive cache: write through
[ 2950.063701] sd 4:0:0:0: [sdb] Attached SCSI disk

How can I solve?

My release is:
Description: Ubuntu 11.10
Release: 11.10

Package friendly-recovery is: 0.2.18

Thank you,
Andrea